The patent documents JP-A-58-155527 and JP-A-58-212631 contain the description about a focus servo for focusing laser light on a signal face of a disc such as CD-ROM for the purpose of reading data from the signal face. For instance, based on a focus error signal obtained from an output of a photo detector which detects reflection light from a disc using four quadrantal detector elements, a feedback loop is utilized to move up and down an objective lens of an optical pickup while keeping constant the distance between the objective lens and the disc whose surface is wobbling with rotation owing to a warp of the disc, whereby the laser light is focused on the signal face.